does tribocharging of different-sized particles cause lightning?

10Aug12

Flying into Washington at night, I noticed lots of bright flashes outside the plane – lightning! I thought about some experiments where particles of a granular material bounce off each other as they are poured out of a hopper. As the particles collide, they acquire an electrical charge which depends on the size of the particle: the bigger particles become positively charged while the smaller particles become negatively charged. This phenomenon is a sort of tribocharging, which means that rubbing two things together (balloons + hair, socks + carpet) cause electrons to move from one object to another. Would this mechanism also cause ice particles in clouds to charge during thunder storms?

Nobody really understands why small particles in a dry granular flow charge negatively while large particles charge positively. One suggestion is that electrons can be transferred from the surface of one insulator to another surface of the same material if a lower energy state is available on the second surface. If the electron on the first surface is already in a low-energy state, it won’t move to the second surface, but a high-energy electron on the second surface will move to another low energy state on the first surface, so that charge is transferred to the first surface. The result after a few collisions is that a larger fraction of electrons on the surface of the smaller object are in low-energy states and are reluctant to leave, so negative charge builds up on the smaller particles.

But what happens in a thundercloud?

Charge separation appears to require strong updrafts which carry water droplets upward, supercooling them to between -10 and -40 °C (14 and -40 °F). These water droplets collide with ice crystals to form a soft ice-water mixture called graupel. Collisions between ice crystals and graupel pellets usually result in positive charge being transferred to the [smaller] ice crystals, and negative charge to the [larger] graupel. [NASA lightning primer]

But the transmission of electrons from the smaller particles to the larger particles is the opposite of what is observed for dry granular powders, where excess electrons end up on the small particles! So how is the ice-graupel system different from a dry powder?

In another surprise, further meteorological experiments indicate that the direction of the charge transfer between the graupel and the ice crystals depends on the temperature:

Back in the laboratory, Takahashi studied the physical reason for the sign change at –10ºC. As graupel warms above –10ºC, it forms a liquid coating. This allows ice crystals to “steal” negatively charged hydroxyl radicals (OH) from the surface of the graupel, making the ice crystals negative and the graupel positive. At temperatures lower than –10º, the graupel surface becomes solid. When ice crystals bump into the solid graupel, branches break off the ice crystal and free hydrogen ions are formed that move from the warmer graupel side to the colder ice crystal side, giving the ice crystals now a positive charge and leaving the graupel negatively charged. reference

So unlike the granular powder, where the small and large particles have the same microscopic structure, the graupel and ice crystals don’t have the same microscopic structure: the ice crystals are, well, crystalline, while the graupel has amorphous patches. So tribocharging of different-sized particles doesn’t appear to play a role in lightning afterall, but maybe the differences between graupel and ice crystals can help to understand why the large and small particles in a dry powder accumulate electric charges with the opposite sign.